The magnetocaloric effect, or MCE, is a property of certain materials, as a result of which property those materials change their temperature when subjected to a change in the magnetic field in which they operate. A related effect known as the electrocaloric effect, or ECE, occurs when certain polar materials, such as ferroelectric polymers are exposed to high electric field strengths. Materials with a “positive” MCE or ECE get hotter as the field increases, and materials with a “negative” MCE or ECE get colder as the field increases.
The MCE and the ECE both are now the subject of intensive research and development to produce refrigerators that employ the effect, using materials that demonstrate large MCE or ECE response as solid refrigerants. Such refrigerators hold the promise of greater thermodynamic efficiency than vapor compression refrigerators, and the elimination of use of such potentially harmful chemicals as CFC's for the working vapor compression fluid.
This process works efficiently, since the release and absorption of heat with the change in magnetic or electric field is a nearly reversible process. However, because the absolute value of the temperature change due to this effect is small compared to the typical requirements for commercially useful refrigeration, there exists a need, as described, for example, in U.S. Pat. No. 4,069,028 by Brown for a method to physically separate the processes of absorbing heat at cold temperature and rejecting heat at hot temperature.
An active regenerator is the means by which scientists and engineers have overcome the low magnitude of the temperature change due to the MCE. U.S. Pat. No. 4,332,135, for example, by Barclay et al. teaches the use of an active regenerator to apply Brayton, Stirling, Ericson, and Carnot cycles by application of an active regenerator. Heat is taken in to the active regenerator at the cold side from the cold side heat exchanger, and heat is removed from the active regenerator at the hot side, through a hot side heat exchanger. Although researchers are only beginning to build devices for ECE applications, there are many different designs so far reduced to practice for MCE refrigerators, some with single active regenerator units or multiple active regenerator units; some move the active regenerator into and out of a stationary magnetic field; some move the magnet instead and keep the active regenerator stationary. Several rotary designs exist, including one described by Brayton in U.S. Pat. No. 4,408,463 in which heat exchange fluid moves radially through a regenerative magnetic wheel, and a more recent design by Zimm et al. as described in U.S. Pat. No. 6,526,759 in which the heat exchange fluid moves circumferentially through the wheel as it rotates through a stationary permanent magnetic field. In U.S. Pat. No. 4,507,928 Johnson describes the use of a continuously reciprocating displacer with at least a pair of active regenerator segments driven into and out of a magnetic field. There are a few designs in which the magnet field source—which could be a conventional or superconducting electromagnet or a sophisticated permanent magnet assembly such as described in U.S. Pat. No. 5,182,914—does not move physically with respect to the stationary active regenerator, but instead increases and decreases the magnetic field amplitude.
One significant drawback to the conventional methods of using MCE in an active regenerator is that a physically large magnetic field must be employed to raise or lower the entire temperature profile along the whole active length of said active regenerator. Because the magnetic fields needed to produce a useful effect are quite powerful, the cost, size and weight of the magnet in a typical magnetic refrigerator design can be the dominant element in the overall cost, size and weight of the refrigerator. Thus it is one object of the instant invention to provide a design for a magnetic refrigerator that minimizes the size and cost of the magnet or magnets required to obtain the desired cooling power. It is a further object of the instant invention to provide a magnetic refrigerator that can achieve a greater transfer of heat per cycle of the refrigerator than is possible in prior art designs. It is a further object of the instant invention to provide a magnetic refrigerator that can attain a larger temperature differential between the cold side heat exchanger and the hot side heat exchanger than is possible in prior art designs.